I don't think there's a pure Perl solution... However, you could try to intercept the underlying execvp library function call (at least on unix-ish systems — I think Windows uses spawn). Then, in your wrapper function you could decide what to do, e.g. return fake results, or just pass through to the real function...  I know this is highly platform dependent, ugly and probably otherwise tricky in a number of ways... but anyhow, these would be the basic steps — just to illustrate the idea... (quite a number of issues remain to be solved!)

Demo perl program

#!/usr/bin/perl

system "/bin/echo", "hello";
# or
# system "/bin/echo hello";
# or 
# my $out = `/bin/echo hello`;
[download]

This is the sample wrapper library, which does nothing except printing a message to stderr, and then forwarding the call to the original library function:

/* fakesys.c */
#include <unistd.h>   /* for execvp */
#include <dlfcn.h>    /* for dlsym */
#include <stdio.h>

static int (*real_execvp)(const char *file, char *const argv[]);

int execvp(const char *file, char *const argv[]) {
    char *msg;
    
    /* lookup original shared lib function */
    real_execvp = dlsym(RTLD_NEXT, "execvp");   /* see dlsym(3) */
    if ( (msg = dlerror() ) != NULL ) {
        fprintf(stderr, "dlopen of execvp() failed : %s\n", msg);
    }
    
    fprintf(stderr, "calling execvp(\"%s\", ...)\n", file);
    
    /* execute original lib function */
    return real_execvp(file, argv);
}
[download]

Compile a shared lib (on Linux):

$ gcc -D_GNU_SOURCE -fPIC -shared fakesys.c -o libfakesys.so
[download]

and call the program with the intercepted execvp call, using the LD_PRELOAD mechanism (—> "man ld-linux.so"):

$ LD_PRELOAD=./libfakesys.so perl ./685699.pl
[download]

Output:

calling execvp("/bin/echo", ...)
hello
[download]

You get the idea... :)